How to handle backorder

Hello All,

I am developing a small DB for a re-seller of commercial products. Customers call the User to order products, the User enters an invoice in the DB and then orders the products from the Manufacturer. (No inventory is kept on-hand, either physically or in the DB) The User receives the products from the Manufacturer weekly but with some items backordered. I've already handled subtracting the B/O items on the original invoice. When the B/O items finally arrive, how do I script an invoice maintaining the Orders::_PK that is already half billed/paid?

My first thought is to create a B/O Table using ORDERS::_PK and Order_Line_Items::Backorder to create a new Invoice that mimics the original. But then what would happen if, say, the User has 5 Widget01's B/O'd and only 3 arrive...... ? I may have bit off more than I can chew here!

Any suggestions are appreciated.